This invention relates to high impact copolyesters.
Homopolyesters of ethylene glycol and terephthalic acid are well known in the art and have been used extensively throughout the world in the production of fiber. Such polymers have not proven satisfactory for most ordinary molding applications for a number of reasons including relatively low impact strength. It has been proposed to produce improved polyesters by utilizing a combination of terephthalic acid and homoterephthalic acid as the acid component. However, homoterephthalic acid is not a widely produced material and is substantially more expensive than terephthalic acid. It has also been proposed to utilize poly(butylene terephthalate) since it has a higher rate of crystallization and thus gives a shorter molding cycle. However, poly(tetramethylene terephthalate) does not have the outstanding impact strength which would be desired. It is well known that with many classes of plastic materials, copolymers tend to have better impact strength than the corresponding homopolymers. Consequently, there are numerous suggestions in the art broadly proposing copolymers utilizing two or more diols. However, such copolymers tend to have poor resistance to hydrolysis.